Digital watermarking is a process for modifying physical or electronic media to embed a machine-readable code into the media. The media may be modified such that the embedded code is imperceptible or nearly imperceptible to the user, yet may be detected through an automated detection process. Most commonly, digital watermarking is applied to media signals such as images, audio signals, and video signals. However, it may also be applied to other types of media objects, including documents (e.g., through line, word or character shifting), software, multi-dimensional graphics models, and surface textures of objects.
Digital watermarking systems typically have two primary components: an encoder that embeds the watermark in a host media signal, and a decoder that detects and reads the embedded watermark from a signal suspected of containing a watermark (a suspect signal). The encoder embeds a watermark by altering the host media signal. The reading component analyzes a suspect signal to detect whether a watermark is present. In applications where the watermark encodes information, the reader extracts this information from the detected watermark. Several particular watermarking techniques have been developed. The reader is presumed to be familiar with the literature in this field. Particular techniques for embedding and detecting imperceptible watermarks in media signals are detailed in the assignee's U.S. Pat. Nos. 6,122,403 and 6,614,914, which are hereby incorporated by reference. Examples of other watermarking techniques are described in U.S. patent application Ser. No. 09/404,292, which is hereby incorporated by reference. Additional features of watermarks relating to authentication of media signals and fragile watermarks are described in U.S. Pat. Nos. 6,574,350 and 6,636,615, and U.S. Patent application Ser. No. 60/198,138 and 60/232,163, which is hereby incorporated by reference.
The problem with video watermarking is that many static image based watermark systems or static watermarking systems have been adapted to video, where “static” refers to processes that do not account for changes of multimedia content over time. However, video is dynamic with respect to time. For example, a mostly invisible image watermark may be visible in video because as the image changes and the watermark remains the same, the watermark can be visibly perceived. In other words, the problem is that the watermark may be mostly invisible in each frame, but the motion of an object through the stationary watermark makes the watermark visible in video. Similarly, an invisible watermark in a video may be visible in each frame, just as artifacts due to lossy compression are imperceptible in video, yet visible if individual frames of the video are examined as still images. It is believed that our eyes and brain average these effects over time to remove the distortion.
This disclosure provides a method of embedding a digital watermark into a video signal using a time-based perceptual mask such that the digital watermark is substantially imperceptible in the video signal. In other words, the watermark is reduced in value where it can be perceived due to the dynamics of video as described above. A digital watermark embedder computes a time based perceptual mask comprising gain values corresponding to locations within a frame. The gain value for a location in the frame is changed as a function of the change in one or more pixel values at the location over time. The embedder uses the gain values of the time based perceptual mask to control embedding of corresponding elements of a digital watermark signal such that the perceptibility of the elements of the digital watermark signal is reduced in time varying locations of the video signal. This inter-frame time-based gain coefficient can be combined with intra-frame spatial-based gain coefficients that make watermarks mostly invisible in each frame based upon static-image perception, or less visible in each static frame and completely invisible in video based upon spatial video perceptual theory or experimentation.
An alternative method is to segment objects and have the watermarks move with each object, labeled object-based masking. The segmentation must be accurate to alleviate edge effects. This method may be very applicable with MPEG-4 where the video is stored as individual objects.
One aspect of the invention is a method of embedding a digital watermark into a video signal such that the digital watermark is substantially imperceptible in the video signal. The method computes a watermark signal corresponding to locations within a frame, where the watermark signal is mapped to locations in the video frame and is computed based upon attributes of the video within the frame. The method varies the strength of the watermark signal over time. The process of varying the strength includes reducing the strength of the watermark signal to make the digital watermark less perceptible in the video in locations where the video has time varying properties. The method embeds the watermark signal into the video at the locations with the varying strength.
Another embedding method computes a watermark signal corresponding to locations within the video, where the watermark signal is mapped to locations in the video and is computed based upon attributes of the video within the frame. The method varies the strength of the watermark signal over time, the varying including selectively reducing the strength of the watermark signal relative to a reference value to make the digital watermark less perceptible in the video in locations where the video has time varying properties that otherwise cause the watermark signal to be more perceptible. The method embeds the watermark signal into the video at the locations with the varying strength.
Further features of the invention will become apparent from the following detailed description and accompanying drawing.